Means have already been applied in conventional electric cells containing aqueous electrolyte to observe the state of charge of the electric cell. They consist in providing at least one of the electrodes with a plurality of active materials which discharge successively at voltage plateaus which are close to but distinct from one another. Thus, French Pat. No. 1 151 414 uses zinc, cadmium and iron to constitute the negative electrode in a pilot storage cell which is charged and discharged at the same time as the battery whose state of charge is to be determined. The disadvantage of such a solution is that a foreign active material is added which can impair the operation of the electric cell.
Further, in U.S. Pat. No. 3,891,458, it is recommended to add a dissolved substance to a primary cell whose cathode active material and electrolyte solvent are thionyl chloride, said dissolved substance being chosen from the group consisting mainly of phosphoryl chloride, sulfolane and sulphur dioxide. However, the examples quote only dissolved substances in concentrations that are too low to provide a second discharge plateau (for phosphoryl chloride, the concentration provided for is 1.5 moles per liter, i.e. 12% by volume).
Preferred embodiments of the present invention mitigate these drawbacks.